Fire retardant unsaturated polyester (UPE) resins with acceptable UV/weathering stability are prepared by reacting dibasic acids, e.g., maleic anhydride and phthalic anhydride, with brominated glycols or alcohols, e.g., dibromoneopentyl glycol (DBNPG), in combination with non-halogenated glycols or alcohols. The dibasic acids react with the glycols and alcohols in an esterification reaction process. However, during the esterification process a small amount of hydrogen bromide (HBr) is generated from the decomposition of the brominated glycols and alcohols, and results in the UPE resin having a dark orange color. Although the orange color is partially bleached out as the UPE resin is thinned, i.e., blended, in vinyl monomers, e.g., styrene, methyl methacrylate, etc. the HBr in the UPE resin reacts across the vinyl group of the vinyl monomer. The resulting brominated compounds, e.g., bromostyrene, are very unstable to heat and readily eliminate bromine during free-radical cure of the UPE resin. Also, bromine is released over time in the cured commercial product, especially if the product is exposed to sunlight and/or heat.
DBNPG is a commercially available fire retardant introduced by Dow Chemical that is used to make fire-retardant polyester resins. DBNPG is characterized by an aliphatic neopentyl structure and has no hydrogens atoms on carbon atoms adjacent to carbon atoms that contain bromine. As a result, DBNPG is relatively resistant to dehydrobromination at elevated temperatures, i.e., at esterification temperatures of about 160° C. to 210° C., compared to other halogenated glycols. DBNPG is also expected to be relatively stable to photodegradation for the same reason.
U.S. Pat. No. 4,420,416 describes that the addition of small quantities of an epoxide such as epichlorohydrin or the polyglycol diepoxide (D.E.R.®736) to UPE resin lightens the dark orange colored resin. However, this process generates new aliphatic bromine compounds that suffer from the same instability to heat and light as brominated vinyl monomers. As a result, the laminates made from the process exhibit extensive discoloration over an extended period of time.
UK Pat. No. 2,065,685 teaches the addition of at least one tri- or pentavalent compound of a Group V element, such as triphenyl antimony or triphenyl phosphite to the UPE polyester resin to improve color. However, UPE resins processed at conventional polyesterification temperatures with these compounds still provide a UPE resin with a dark orange color.
U.S. Pat. No. 4,175,072 teaches the inclusion of from 5 to 85 mole percent of tetrahydrophthalic acid or anhydride, or endomethylene tetrahydrophthalic acid or anhydride, into the UPE polyester resin for color improvement. However, UPE resins processed at conventional polyesterification temperatures with these compounds still provide a UPE resin with a dark orange color.
The discoloration over time of cured laminate compositions containing brominated UPE resins, e.g., construction panels and skylight fixtures, remains a problem for the industry. The use of DBNPG has helped to some extent, however for the reasons stated, current UPE resins made with DBNPG and other brominated glycols still experience significant discoloration over time. As a result, new UPE resins containing bromine are needed.